Nerve growth factor (NGF) is a protein which has prominent effects on developing sensory and sympathetic neurons of the peripheral nervous system. NGF acts via specific cell surface receptors on responsive neurons to support neuronal survival, promote neurite outgrowth, and enhance neurochemical differentiation. NGF actions are accompanied by alterations in neuronal membranes (Connolly et al., 1981, J. Cell. Biol. 90:176; Skaper and Varon, 1980, Brain Res. 197:379), in the state of phosphorylation of neuronal proteins (Yu, et al., p.1980, J. Biol. Chem.255:10481; Haleqoua and Patrick, 1980, Cell 22:571), and in the abundance of certain mRNAs and proteins likely to play a role in neuronal differentiation and function (Tiercy and Shooter, 1986, J. Cell. Biol. 103:2367).
Forebrain cholinergic neurons also respond to NGF and may require NGF for trophic support. (Hefti, 1986, J. Neurosci., 6:2155). Indeed, the distribution and ontogenesis of NGF and its receptor in the central nervous system (CNS) suggest that NGF acts as a target-derived neurotrophic factor for basal forebrain cholinergic neurons (Korsching, November/December 1986, Trends in Neuro. Sci., pp 570-573).
While a number of animal homologues to NGF have become known, it was not until recently that an apparently distinct nerve growth factor was identified that nonetheless bears some homology to NGF (Leibrock et al., 1989, Nature 341:149). This factor, called brain-derived neurotrophic factor (BDNF), now also called NT-2, was purified from pig brain, and a partial amino acid sequence determined both from the N-terminal end and from fragments purified after cleavages. The longest sequence, compiled from several overlapping fragments, was used to synthesize two sets of oligonucleotides that were used to prime the amplification of a pig genomic template using the polymerase chain reaction (PCR). The nucleotide sequence between the two primers was determined and used to synthesize specific primers for further PCRs on a complementary DNA template obtained by reverse transcription of total RNA isolated from the superior colliculus of the pig brain. The nucleotide sequence so obtained contained an open reading frame coding for a protein of 252 amino acids, starting with the first methionine codon found after four in-frame stop codons. Leibrock, et al. speculate that there is no reason to think that BDNF and NGF should be the only members of a family of neurotrophic proteins having in common structural and functional characteristics, and the authors hope that these common structural features could be used to aid the discovery of other members.
More recently, another novel neurotrophic factor closely related to .beta.NGF and BDNF was discovered, called neuronal factor (NF), or neurotrophin-3 (NT-3). (Hohn, et al., 1990, Nature 344:339; Maisonpierre, et al., 1990, Science 247:1446; Rosenthal, et al., 1990, Neuron 4:767. Both BDNF and NT-3 share approximately 50% of their amino acids with .beta.NGF. High levels of mRNA coding for BDNF and NT-3 occur in the adult rodent brain. .beta.NGF, BDNF, and NT-3 support survival of selected populations of chick sensory neurons, suggesting independent roles in the regulation of neuronal survival during development.
Neuronal survival and growth is also affected by growth factors for non-neuronal cells, including fibroblast growth factor (FGF), epidermal growth factor, and insulin-like growth factors. (Morrison, et al., 1987, Science 238:72; Walicke, 1988, J. Neurosci. 8:2618; Bhat, 1983, Dev. Brain Res. 11:315). Basic FGF (bFGF) supports initial survival and subsequent fiber outgrowth of dissociated rodent fetal neurons in culture. While neurons from many brain regions are affected, the proportion of neurons surviving varies among brain regions, suggesting that subpopulations of neurons are responsive to bFGF. (Morrison, et al., 1986, Proc. Natl. Acad. Sci. 83:7537; Walicke, et al., 1986, Proc. Natl. Acad. Sci. USA 83:3012). Since bFGF lacks a signal sequence typical for released proteins, and since bFGF levels present in the brain are much larger than those of .beta.NGF and BDNF, it has been questioned whether bFGF plays a physiological role as neurotrophic factor and has been proposed that bFGF acts as "injury factor" released in events involving cellular destruction. (Thoenen, et al., 1987, Rev. Physiol. Biochem. Pharmacol. 109:145).
Another neurotrophic factor having potential therapeutic use for peripheral nervous system disorders, ciliary neurotrophic factor (CNTF), has been cloned and expressed. (Lin, et al., 1989, Science, 246:1023). CNTF, which was purified from adult rabbit sciatic nerves, acts on the peripheral nervous system and appears to be completely unrelated to NGF.
It is an object to identify a fourth neurotrophic factor in the NGF family and to obtain nucleic acid encoding such a factor.
It is another object to synthesize such a new factor in recombinant cell culture.
It is yet another object to provide variants and modified forms of such a new factor.
It is an additional object to prepare immunogens for raising antibodies, as well as to obtain antibodies, capable of binding such a new factor or variant or modified form thereof.
Another object is to provide diagnostic and therapeutic compositions comprising such a new factor or variant or modified forms thereof, and methods of therapeutic treatment.